This invention relates to a photodiode which improved sensitivity and is suitable for integration with related semiconductor devices or circuits in bipolar ICs.
In a conventional photodiode which is fabricated by a standard bipolar process, a silicon substrate of first-type (e.g. p-type) conductivity and an epitaxially grown layer of opposite conductivity (e.g. n.sup.- -type) provide a PN junction. In a selected region the epitaxial layer is converted to a p.sup.+ -type diffused isolation region which is in such a pattern that an n.sup.- -type island region is defined on the silicon substrate. In the surface of a marginal area of the island region there is formed an n.sup.+ -type emitter diffusion region, which is used as cathode while the p.sup.+ -type diffused isolation region is used as anode. A field oxide film lies over the entire area of the device except where aluminum interconnections are formed. In operation of this diode the PN junction between the p-type silicon substrate and the n.sup.- -type epitaxial layer in the island form is reverse-biased. When the diode is illuminated a current will flow between the anode and the cathode.
In the conventional photodiode of such construction the thickness of the epitaxially grown layer is usually about 10 .mu.m, and it is difficult to greatly reduce this thickness. That is, the PN junction of the photodiode is at a considerable depth from the surface on which light is incident. Therefore, electron and hole pairs created in regions close to the surface do not effectively contribute to the output current, so that the sensitivity of the photodiode is not sufficiently high.
In the case of integrating a photodiode with related circuit elements in a semiconductor chip to produce a photodetector, low sensitivity of the photodiode causes the circuit elements to be made complicated and to occupy a considerably large area of the semiconductor chip and, therefore, leads to high production cost. Furthermore, the resultant photodetector suffers from relatively high probability of wrong function because it is difficult to secure ample S/N ratio due to insufficient sensitivity of the photodiode. In industrial fabrication the amount of unacceptable products becomes considerable.